feat(ui): Phase 2 C3 — Settings 页 + Dialogs + 窗口状态保存

- src/storage.rs: AppState (last_window_width/height) + load/save
  atomic write 到 %APPDATA%/WindowsGotify/state.json（§18）
- ui/settings-page.slint: 5 分组（Connection / Notifications / Startup
  / Appearance / Advanced），Appearance 实时切主题（System/Light/Dark
  radio）；Advanced 折叠
- ui/dialogs.slint: AboutDialog + ConfirmDialog（§54）
- ui/server-card.slint: 单 server 行（name/url/enabled toggle/edit/remove）
- ui/app.slint: settings 页切换、About 导航、theme 联动
- src/lib.rs: load_state → 启动时应用窗口尺寸；ui.window()
  .on_close_requested 在 OS 关闭前 save_state；wire about/close
  callbacks
- src/error.rs: 加 AppError::Storage(String) 变体
- Cargo.toml: 加 serde + serde_json

避坑：
- Slint 1.x 同文件内 export component 必须在引用前定义（sub-components
  移到 SettingsPage 上面）
- Slint 不支持 padding 两值简写 (padding: 0 16px) → 用 padding-* 四属性
- Slint 不支持 Rectangle 内建 border-color 覆盖 → 命名 card-border
- Slint 不支持 alignment: right → 用 spacer + horizontal-stretch: 1
- 修了一处 .slint 文件被 PowerShell Set-Content GBK 损坏的问题（已
  通过 write 工具 + UTF-8 no BOM 重写并清理 ui/ 所有 .slint）

验证：cargo build / build --release 都 exit 0，0 warning
- debug   27.55 MB, subsystem = WINDOWS_CUI
- release  8.10 MB, subsystem = WINDOWS_GUI

Cargo.toml

@@ -34,6 +34,10 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 # --- Error handling ---
 thiserror = "2"
 
+# --- Serialization (Phase 2 C3 storage) ---
+serde     = { version = "1", features = ["derive"] }
+serde_json = "1"
+
 # Slint's `.slint` files are compiled by `build.rs`; see build.rs.
 [build-dependencies]
 slint-build = "1"